High-efficiency metal membrane element, filter, and process for making

ABSTRACT

A high-porosity metallic membrane element comprising a sintered element having at least about 55% porosity, the sintered element comprising a matrix of substantially interconnected pores, each of the pores being defined by a plurality of dendtritic metallic particles. A preferred form is made from pure nickel, preferably filamentous nickel powder. The high-porosity metallic membrane element, comprising the aforementioned sintered element having at least about 55% porosity, can be sealed within a filter housing to produce a highly porous filter device with a filtered fluid flow path through the metal membrane element. Also disclosed is a method of making the high-porosity metallic membrane element which includes depositing by air-laying techniques a substantially uniform low-density bed of a sinterable dendritic material into a mold suitable for applying compressive force thereto, compressing the low-density bed of sinterable dendritic material to form a green form, and sintering the green form. The present filter devices exhibit superior porosity and face velocities, negligible outgassing and limited particle shedding when compared to the devices of the prior art.

This is a continuation of application Ser. No. 08/071,554 filed on Jun.4, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a fluid filter and a processfor making it. In particular, the invention is a novel all-metal gasfilter with high efficiency and low outgassing characteristics that isuseful as a point-of-use filter for semiconductor process gases.

2. Description of the Prior Art

Semiconductor manufacturing is constrained by the limitations of purity.In chemical vapor deposition of the alternating layers of silicon anddopant, a critical aspect of the process involves the absence of anyparticulate impurities. The presence of a minute particle can destroy anentire silicon wafer representing many dollars of potential end-product.To that end, an entire industry has developed concerned with only onething--the filtering of the gases that may come into contact withsemiconductor product during its formation.

Clean rooms equipped with HEPA (High Efficiency Particulate Air) filtersare the first line of defense. Process equipment is located within"clean rooms" that are filled with carefully filtered air. The design ofthe equipment itself endeavors to minimize particle shedding,outgassing, and contamination from the materials used to transport anddeliver high-purity gases such as silane, arsine, hydrochloric acid andphosphine. An important component in these delivery systems is thefilter which insures that particulate contamination does not reach thepoint where the gas is discharged onto the work (point of use). Thesefilters must not only remove any particulate material, but also must notadd any gaseous contamination to the high purity gases. In addition, thegas delivery systems must also be as compact as possible to eliminatecontamination, both particulate and gaseous, which might arise fromeither the installation of such systems, or the normal wear associatedwith usage. This is especially the case with highly corrosive gases suchas hydrochloric acid. Therefore the filters must not only removeparticulate material and not be a contributor of gaseous impurities, butthey must also be as compact as possible and have small internal andfilter volumes.

Various filters are used for filtration of such gaseous fluids to insureultra high levels of purity in terms of particulate contamination. Theseinclude: organic membrane filters, ceramic filters, filters formed fromporous metal structures and filters formed from metal fibers. Althoughsome of these various filter media are capable of providing particulatecontamination control to levels less than one part per million orgreater in terms of particulate control, they are characterized by largefilter areas. Due to the large flow area required to sustain flow atreasonable pressures and maintain low face velocities to insureparticulate retention, gaseous impurities such as moisture, oxygen andespecially hydrocarbons are often present in detectable levels (partsper million). This contamination can occur during manufacture of thefilter, during installation of the filter when it is exposed to anatmosphere other than a high purity gas, or even as a result ofoutgassing from the material the filter is packaged in. In addition,large filter volumes require relatively larger housings to contain them.This in turn results in a greater likelihood of contamination due bothto installation and usage and the need for larger gas delivery systemsto fit the filters.

Present metal filters include stainless steel, nickel, or nickel alloysintered-powder types such as the Wafergard® II SF ( MilliporeCorporation, Bedford, Mass.), and the Mott GasShield (Mott MetallurgicalCorporation, Farmington, Conn.) (See U.S. Pat. No. 5,114,447, Davis)line of filters. Such filters, being all metal, exhibit low outgassing,high efficiency, corrosion and temperature resistance, low porosity andgas throughput, and high structural strength. However, low porosity hascontinued to be a drawback for typical sintered metal powder filterelements. Porosities for the above filters range from 40 to 44%,limiting the flow-through characteristics of these filters. The lowporosities are inherent in the processes used to manufacture sinteredmetal powder filters. Typically, the powders are compacted into a moldto form a "green form," then sintered to join the metal particlestogether to impart the necessary strength. The final filter elements (or"membranes") may be cut from a flat sintered sheet of metallic powder,or molded into the final shape in the molding step. The temperatures atwhich the sintering proceeds are a critical factor in determining thefinal porosity. Higher temperatures lead to increased strength, butlower porosity; lower temperatures lead to decreased strength and highporosity. Until now, the final porosity was limited to about 45% in thesintered metal powder art.

There exists a need for increased porosity and gas throughput in themetal filter art. Increased porosity would allow for the construction ofsmaller filters with all the positive aspects of highly porous metalfilters, with less outgassing and particulate shedding problems.

SUMMARY OF THE INVENTION

In accordance with the present invention it has been found that highlyporous, high-flow filter devices made front filamentary metal powderscan be made which require a fraction of the filter volume found inexisting filters. The filters of the present invention retain the highlevel of particulate efficiency (less that one part per million passagethrough the filter) found in existing filters, while operating atexponentially higher face velocities. In addition, the gas pressurerequired to sustain these velocities is equivalent to or less thanexisting filters. The filter of the present invention has minimalinternal volume and is highly compact, resulting in very low contaminantgeneration due to exposure to atmospheres other than high purity gasesand the wear associated with usage. In addition, the present inventionshares other advantages characteristics of porous metal filters, namelygood mechanical and thermal properties allowing operation at elevatedtemperatures and high differential pressures, and the absence ofparticulate shedding associated with prolonged usage.

The present invention is a high-porosity metallic membrane element,comprising a sintered element having at least about 55% porosity, thesintered element comprising a matrix of substantially interconnectedpores, each of the pores being defined by a plurality of dendriticmetallic particles. A preferred form of the invention is made fromdendritic nickel powder and has a porosity in excess of 65%. In apreferred embodiment a low sintering temperature of from about 675degrees to about 725 degrees centigrade is employed. No binder isemployed, practically eliminating outgassing in the final membraneelement.

The invention also includes a high-porosity metallic membrane filterdevice. The filter includes a sintered membrane element having at leastabout 55% porosity, the sintered element comprising a matrix ofsubstantially interconnected pores, each of the pores being defined by aplurality of dendritic metallic particles. The filter also includes afilter housing defining a fluid conduit, the conduit being a casing forretaining the filter element in the fluid flow path, the casing having afront and a back with the element being located between them. In apreferred embodiment, the filter element is welded to the casing wall.Means for sealably connecting the casing to a fluid to be filtered areprovided by utilizing typical Swagelock®-style connectors.

Another aspect of the invention is a method of making a high-porositymetallic membrane element comprising the steps of first depositing asubstantially uniform low-density bed of a sinterable dendritic materialinto a mold suitable for applying compressive force thereto. The secondstep is compressing the low-density bed of sinterable dendritic materialto form a green form. The last step is sintering the green form at atemperature below the melting point of the metallic material. In apreferred method of the invention, depositing the substantially uniformlow-density bed includes, but is not limited to, the steps of firstair-laying the sinterable dendritic material into a mold suitable forapplying compressive force thereto, the air-laid bed having a densityless than or equal to the density of the sinterable dendritic material.In a preferred embodiment, the sinterable dendritic material isfilamentary nickel powder. Compressing the bed of nickel is accomplishedat a pressure below about 500 psi, in the absence of any binder. Thesintered green form is compressed at a second higher pressure, therebyimparting additional structural rigidity to the element. The higherpressure is generally in the range of 600-1000 psi. The final porosityof the sintered membrane element is generally from about 55% to about70%.

Accordingly, it is an object of the present invention to provide ahighly efficient high-porosity metallic membrane element useful as apoint-of-use filter in the semiconductor industry. The high efficiencyof the invention allows a substantial savings in space utilized in theprocess instrumentation, reduced outgassing due to decreased internalsurface area, and reduced pressure drop over the length of the filter.

It is another object of the invention to provide a method for forminghigh-porosity metallic membranes with excellent structural strength,very high porosity, and without the use of any binders.

It is still another object of the invention to provide a membraneelement that may be formed in any of numerous shapes for inclusion intoa filter housing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is an photomicrograph of the surface of a sintered nickelmembrane element made in accordance with the present invention. A 10 μmscale is provided for purposes of illuminating the relative sizes of thepores.

FIG. 1(b) is an photomicrograph of the surface of a prior art sinterednickel membrane element, taken under the same conditions as FIG. 1(a).Pore sizes appear similar, but the apparent pore density is visiblylower.

FIGS. 2(a-f) are schematic representations of the process for producinga metallic membrane element. FIG. 2(a) depicts the air-laying step andapparatus used therein. FIG. 2(b) shows the second step of low-pressurecompression of the air-laid powder into a green form. Porosity (PO) ofthe green form is about 80-90%. FIG. 2(c) depicts the high-temperaturevacuum bake/hydrogen reduction, resulting in a sintered green form witha PO ranging from 70-80%. FIG. 2(d) shows the second compression at ahigher pressure, typically from 600-1110 psi. PO decreases to between55-70%, FIG. 2(e) shows the cutting step. Wire EDM cutting is depicted.FIG. 2(f) shows the final low-temperature vacuum bake step.

FIG. 3 is a graph of total hydrocarbons (ppb) versus time (minutes)representing total hydrocarbon outgassing from the Mott GasShield™ modelPOU-05 and the present invention.

FIG. 4 is a graph of face velocity (slpm/cm²) versus the log reductionvalues (LRV) for the invention, and for various prior art products. Theinvention (labeled "nf") shows higher LRVs at all face velocities. Priorart products are the "sf" filter (Wafergard® II SF Mini In-line GasFilter, Millipore Corporation, Bedford, Mass.); Ultramet-L Gaskleen™,model #GLFF4000VMM4, (Pall Corporation, East Hills, N.Y.); MottGasShield model POU-05 (Mott Mettalurgical Corporation, Farmington,Conn.).

FIG. 5 is a graph plotting face velocities (slpm/cm²) against pressurefor the same products as in FIG. 4.

FIG. 6 is a graph showing the realtionship between porosity andpermeability of the membrane of the present invention.

FIG. 7 is a cross-sectional schematic of the filter device of thepresent invention prior to welding the membrane element to the innercircumference of the filter housing.

FIG. 8 is a cross-sectional schematic of the filter device of thepresent invention after welding the membrane element to the innercircumference of the filter housing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a high efficiency high-porosity all metal filtermembrane and a method for making it. Filters incorporating the membraneexhibit increased fluid throughput, decreased pressure loss,substantially lower outgassing, and decreased size and complexity. Thesecharacteristics make this filter ideal as a point-of-use process gasfilter in the semiconductor, and related, industries.

The following terms are used in this application. The term "porosity" isdefined as the amount of pore volume per total volume of the membraneelement. Per cent porosity is the pore volume divided by the totalmembrane element volume, multiplied by 100.

As used herein, the term "metallic" is meant to describe allmetal-containing materials. This includes, but is not limited to, puremetals, mettaloids, metal oxides, metal alloys, and similar combinationsthat would be obvious to one of ordinary skill in the metalworking art.

As used herein the term "membrane element" describes the wafer-likeproduct of this invention. It is characterized in that it has a highinternal pore volume, excellent structural rigidity and strength, andlow pressure drop when fluids, specifically gases, are passed throughit. Other shapes may be used in addition to wafers.

As used herein the term "matrix" describes a physical structurecomprising a network of interrelated pores or crevice-like areas. Thematrix, while generally a uniform structure, is not necessarily 100%uniform. Some pores may be completely unconnected to other pores, andthus may not be connected to the matrix. "Substantially interconnected"can thus be interpreted to mean that the majority of the pores willtouch upon or share elements of at least one other pore, therebyallowing communication between the pores.

As used herein the term "dendritic" refers to the tree-like appendagesthat the metallic particles display. The term comes from "dendrite"which is a highly branched nerve cell. The dendritic property of themetallic powder makes it possible to attain a higher pore area than withnon-dendritic metal powders because of the interactions of the brancheswith each other. Other metal powders which are or may be made dendriticcome within the scope of this invention.

As used herein the term "green form" is a term well-known in thesintering art. It refers to the compressed metal powder structure beforeit is sintered. The green form displays a higher porosity than the finalsintered membrane, but is very fragile.

As used herein the term "sinterable dendritic material" refers to anymatter whose individual particles have a highly branched externalsurface, the particle also being capable of being sintered.

As used herein the term "substantially uniform" when used to describethe low-density bed means that there are few or no significant localvariations in the density of the air-laid bed.

With reference now to FIG. 1(a), a close-up photomicrograph of asintered metallic membrane element made in accordance with the method ofthe invention is shown. The membrane depicted is comprised of afilamentary nickel powder. In a preferred embodiment a filamentarynickel powder such as commercially available INCO 255 (Novamet SpecialtyProducts, Wyckoff, N.J.) is used. Applicant has discovered that underthe proper conditions of pressure, temperature and time, a novel andsurprisingly porous material can be fabricated. While the presentinvention is primarily described as employing filamentary nickel powderswith a Fisher size of between 2 and 3 μm (ASTM B 330), it is understoodthat other metal powders having the physical characteristics of thisfilamentary, dendritic powder may be employed to produce highly porousmetal membranes, using the same techniques as described herein.Applicant has discovered that manipulation of certain criticalconditions results in a surprisingly porous yet strong metallic membranestructure. In this regard, other filamentary metal powders may also beused or later developed which may be successfully used in thisinvention. The size of the pores shown in FIG. 1(a) ranges from about 2to about 10 μm. The density of the starting filamentary powdered metalis typically 0.5 to 1.0 g/cc. The final product density is between about2.75 and 3.0 g/cc.

FIG. 1(b) is a similar photomicrograph of a prior art nickel filtertaken under identical conditions. Note that the pore size isapproximately the same as the present invention, but the pore frequencyis visibly less. This is reflected in the fact that the density of theprior art filter is 5.0-5.75 g/cc, while the density of the membrane ofthe present invention is from about 2.75 to 3.0 g/cc.

FIG. 2 is a schematic generally describing the process of making ametallic membrane element. In accordance with the process of the presentinvention, a highly porous metal membrane is prepared by employing avariety of novel operations. Generally, the steps of the process includedepositing a uniform bed of metal powder by air-laying a bed ofsinterable dendritic metallic powder to form a highly uniform,exceedingly low-density powder bed whose apparent density is equal to orless than that of the powder as it is obtained from its container (FIG.2(a)). The next step is the low-pressure press of such a bed to form ahigh-porosity semi-self supporting green form (FIG. 2(b)), and then thelow temperature sintering of the green form to produce a self-supportinghighly porous metal membrane with porosity exceeding 70% (FIG. 2(c)).

FIG. 2(a) depicts the first step of air-laying. As mentioned above, theformation of an exceedingly low-density bed of high uniformity isaccomplished through the use of air-laying procedures. The term"air-laying" as used herein refers to the procedure whereby apre-determined mass of powder 10 is sifted through a screen sieve 20 andallowed to fall under the influence of gravity into a mold 30 below offixed volume. Since the powder is "fluffed up" in this manner, itsdensity becomes lower than that of the packaged powder. The distance thepowder falls before coming into contact with the mold will necessarilyvary depending on the area and shape of the mold. A variety ofindividual molds can be used depending upon the final shape and size ofthe desired product. For example, a 13 cm diameter round mold requires afalling height of at least 25 cm when using filamentary nickel powdersto insure a powder bed 40 of substantially uniform thickness anddensity. A larger diameter mold would require a larger falling distance.One of ordinary skill in the art will be able to determine this heightthrough the use of routine experimentation, given the examples set forthherein. The bed 40, formed in the manner described, has a density equalto or less than that of the apparent density of the powder 10. Apparentdensity is determined following the procedure outlined in ASTM B 329.For filamentary nickel powders with an apparent density of approximately1.0 g/cc the air-laid bed density can be as low as 0.7 g/cc.

Now with reference to FIG. 2(b) the air-laid bed 40 formed as describedis then pressed using compressing means 50 to the desired thicknessusing relatively low pressure, forming a green form 60. The resultingporosity ranges from 80-90%. The degree of pressure required necessarilydepends on three variables, namely the density of the air-laid bed 40,the thickness of the bed, and the desired thickness of the pressed greenform 60. For example, for a bed with a density of 0.8 g/cc and athickness of 0.6 cm, a force of 30 kg/cm² (430 psi) is required toobtain a green form thickness of 0.4 cm. The density of such a greenform would be 1.3 g/cc and have a porosity of 85%. The green form 60 isself-supporting only to the extent that it can, with care, be removedfrom the mold while retaining its structure. However, the introductionof relatively minor stress on green form 60 can cause it to lose itsintegrity.

Now with reference to FIG. 2(c), self-supporting green form 60 is givenadditional strength through the sintering step depicted therein.Generally, sintering is accomplished by heating a metallic powder in asintering oven 70 to a temperature below its melting point in thepresence of an inert or reducing atmosphere, or in vacuum. One ofordinary skill in the an of sintering will be able to determine thespecific atmospheric conditions in which to sinter. The temperature and,to a lesser degree, the duration of the sintering process, are twocritical factors which determine the final dimensions, and henceporosity, of the metal membrane. The porosity of the sintered producttypically decreases to 70-80%. This occurs as a result of both sinterbonding of the powder particles and shrinkage of the membrane. A lowertemperature and shorter sinter duration result in a membrane withsmaller degrees of both sinter bonding and shrinkage. For example, agreen form with a porosity of 80% sintered at 950° C. for 5 minutesresults in a membrane with a porosity of 58%. The same green formsintered at 800° C. for 5 minutes results in a membrane with 72%porosity. It is apparent that a decrease in temperature results in anincrease in resultant porosity. Applicant has discovered that sinteringat even lower temperatures is possible. However, a lower limit totemperature does exist insofar as a given metal will not sinter unlessgiven sufficient heat. The lower limit for sintering for filamentarynickel powder with a Fisher size of 2-3 μm has generally been found tobe between about 500° and 600° C. The preferred form of the inventionutilizes temperatures between about 675° and 725° C., as this results inthe appropriate membrane porosity and dimensions when combined with theproper formation of the air-laid bed and green form. A most preferredform of the invention utilizes a sintering temperature of 675° C.

As shown in FIG. 2(d), the next step is to press the sintered green form80 in compressing means 50 to attain the final dimensions desired. Thepressing is generally done at a higher pressure than the initialcompaction step, given the fact that the sintered form 80 is now muchmore rigid. The pressures generally used in this step are usually fromabout 600 to 1100 psi. In a preferred embodiment, the sintered greenform is pressed at greater than 1000 psi. This step reduces the porosityof the product to its final porosity value to produce membrane elementsheet 90. The final porosity is typically greater than 55%, and in apreferred embodiment greater than 65%.

As shown in FIG. 2(e), the metallic membrane element sheet 90 may be cutinto forms that allow construction of useful filter devices. In apreferred embodiment, wire Electrical Discharge Machining (EDM) isemployed for this purpose. Wire EDM cutting is defined as the cutting ofmetals with a thin wire 100 through which a high electrical current ispassed. This method of cutting the membrane element sheet 90 intomembrane elements has given the best results. However, one of ordinaryskill in the an will be able to adapt other cutting methods that maywork equally well. For instance, cutting with an abrasive wheel or lasermay effect adequate separation. In a preferred embodiment the size ofthe nickel membrane element 110 is approximately 1.2 cm in diameter and0.25 cm in thickness.

Referring now to FIG. 2(f), the membrane elements 110 arelow-temperature vacuum-baked to remove any volatiles introduced duringthe process. The temperature is usually less than 200° C. The finalproduct exhibits minimal outgassing as depicted in FIG. 3. FIG. 3 is agraph of total hydrocarbons (ppb) versus time (minutes). When comparedagainst the prior an, outgassing from the present invention isnegligible. Outgassing was measured on a Beckman Model 400A THCAnalyzer. The analyzer utilizes a flame ionization detector (FID) todetermine the amount of total hydrocarbons (hydrocarbons of allmolecular weights) present in a gaseous sample. In the present case, thegas was passed through the filter and on to the analyzer.

The small-diameter membrane element disks demonstrate high efficiencyand flow rates. As depicted in FIG. 4, a filter made in accordance withthe present method (designated "nf") clearly shows the high efficiencyof a nickel membrane element. FIG. 4 is a graph of face velocity(standard liters per minute, hereinafter "slpm") versus log reductionvalue (LRV). LRV is defined as the log of the ratio of two numbers. Inthe present case, the ratio is that of the number of particles impactingthe filter membrane element on the upstream side of the filter, to thenumber of particles detected downstream of the filter. Therefore, an LRVvalue of 7 would imply a challenge of 10⁷ particles and the detection of1 particle downstream, the log of this ratio being 7. The test isconducted by generating an aerosol containing several million particleswith a size distribution centered around 0.014 m, passing this aerosolthrough the filter and counting those that pass with a condensationnucleus counter (CNC).

The nickel membrane element of the present invention demonstratesexcellent gas permeability and exponential flow characteristics. FIG. 5is a graph of slpm/cm² versus pressure. Compared are the sintered metalnickel powder filter of the present invention ("nf"), one from MottMetallurgical Corporation (Mott GasShield™), a sintered steel powderfilter from Millipore Corporation, Bedford Mass. (model Wafergard® II SFMini Inline Gas Filter), and a steel mesh filter from Pall Corporation(Ultramet-L Gaskleen™, model #GLFF4000VMM4, East Hills, N.Y.). Of thethree sintered metal powder filters, the 63% porous nf filter exhibitssignificantly greater permeability at all operating pressures aboveapproximately 5 psi. At 10 psi, the nf filter has a face velocity of 2.4slpm/cm² while the other two are at 0.8 and 1.5, respectively. At 20 psithe difference is even greater, with the nf filter at 7.5 versus 1.8slpm/cm² for the Mott filter.

FIG. 6 is a further demonstration of the exponential increase in gasflow made possible by the present invention. FIG. 6 is a graph ofpermeabilty in standard liters per minute for a given pressure(slpm/psi) versus per cent porosity. The data points shown arenormalized for a common thickness of 0.10 inch (0.254 cm), having anarea of 1 cm². Typical prior art devices with porosities of 44%(material shown in FIG. 1(b)) have a permeability of approximately 1.2slpm/psi. However, an 11% increase in porosity to 55% (a 25% increase inporosity) leads to a permeability of 2.2 slpm/psi, an 83% increase inpermeability. A 65% porous filter membrane element has a permeability of4.0 slpm/psi, an increase of 233% over the 44% porous membrane. Finally,a 70% porous membrane is 333% more permeable than a 44% porous membrane.The impact of the exponential relationship shown in FIG. 6 isdemonstrated by the increased face velocities for the 63% porous nffilter, as compared to the prior art devices (FIG. 5).

It is clear that the present invention is a clear departure from theprior art. Typical prior art porosities range from 40-44%, withaccompanying low permeabilities. The present invention demonstratespermeabilites of from 83% to 333% greater than those demonstrated bymaterials of the prior art, a surprising result.

As shown in FIGS. 7 and 8, the membrane element 110 may be incorporatedinto a filter device 120. The filter device 120 comprises a filterhousing 130 defining a fluid conduit 140, filter housing 130 comprisinga casing for retaining the element in the fluid flow path, the casinghaving an anterior 132 and a posterior 134 with the element beinglocated therebetween and being sealably joined to the interior casingwall 136 and 138, the housing 130 thereby defining a filtered fluid flowpath. Means for sealably connecting the casing to a fluid to be filteredare provided. These include Swagelok® fittings and the like. The mainrequirement for the connecting means is that it be fluid-tight, i.e.,does not leak. One of ordinary skill will be able to determine whatmeans are appropriate under the particular connecting scenarios.

Fabrication of filter devices incorporating the membrane elements isaccomplished according to the following general procedure. First, asuitable material is selected for the filter housing 130. The materialmust be suitable for use under the ultra-clean conditions necessary inthe semiconductor fabrication art. Stainless steel is a preferredmaterial, although one of ordinary skill in the an is able to select asuitable filter housing material. The housing is usually constructed oftwo symmetric halves 132 and 134. The metal membrane element 110 issituated between the two halves 132 and 134. A lip 145 is machined orcast into the outer surfaces of the two housing halves 132 and 134 priorto assembly. Halves 132 and 134 are then united with membrane 110,touching the inner circumference of the housing as depicted in FIG. 7.The two halves 132 and 134 are welded together by heating or othermeans, sealing membrane 110 within the two halves of the housing. Whenit solidifies, the result is a solid steel-nickel weld bead 150 whichextends from the periphery of the filter housing 160 to the interior ofthe membrane 110 as shown in FIG. 8. Examination of photomicrographs ofsectioned devices have shown this to be the case when correct weldingparameters are followed. One of ordinary skill in the art is able todetermine the welding parameters through routine trial and error.

Having now generally described this invention, the same will becomebetter understood by reference to certain specific examples which areincluded herein for purposes of illustration only and are not intendedto be limiting unless otherwise specified. All patents and publicationscited herein are fully incorporated by reference in their entirety.

EXAMPLES Example 1

Effect of varying the thickness of the air-laid bed on green formdensity

This example shows that differences in the green form density aredirectly related to the amount of compaction selected during the initialcompressing step. To start, three grams of filamentary nickel powder(INCO 255) were sifted into a 2.54 cm diameter mold as previouslydescribed under air-laying techniques, and two individual but identicalgreen forms were made. The initial thickness of both air-laid beds was0.3-0.4 cm. The first mold (green form number 1) was pressed at 150 psito a green form with a thickness of 373 cm and a porosity beforesintering of 82%. The same mold with the same air-laid bed of INCO 255nickel powder is compressed again at 500 psi to a thickness of 0.261 cmand a porosity of 74% before sintering. Both are sintered in vacuum at675° C. for 20 minutes. Number 1 has a final porosity of 75% (a changeof -7%), number 2 a final porosity of 63% (a change of -11%). Clearlythere is a direct relationship between the pressure applied to the INCO255 nickel powder and the final porosity. With this teaching one ofordinary skill in the art is able to determine with routineexperimentation what pressure will be necessary to attain a certaindesired final sintered density.

Example 2

Effect of varying the weight of the green form on the final density

Adding weight (i.e. more powder) to the mold results in a thickerproduct with higher density. Two green forms of different weight butidentical dimensions are made as previously described in Example 1. Thefirst (number 3) had an initial weight of 2.50 g, and an initialporosity of 75% and was pressed at 500 psi. The second (number 4) had aninitial weight of 3.50 g and tin initial porosity of 66% and was pressedat 1000 psi. Both green forms had a thickness of 0.230 cm. Both weresintered at 675° C. for 20 minutes in vacuum. Number 3 had a finalporosity of 66% (a change of -9% from the initial porosity). Number 4had a final porosity of 54% (a change of -12%).

Example 3

Effect of varying the sintering temperature on the density

Four green forms were made to the same specification as 1, 2, 3 and 4 asdescribed above, number 5 having the same weight and dimensions asnumber 1, number 6 as number 2 and so on. All 4 green forms weresintered at 775° C. for 20 minutes. An additional four green forms(numbers 9-12) were made, again as numbers 1-4, and sintered in vacuumat 725° C. for 20 minutes.

The results of the three examples below clearly indicate that both greenform density and sintering temperature have a direct impact on the finalporosity of the metal membrane. It is apparent that the more dense thegreen form, the greater the decrease in porosity due to sintering. It isalso quite clear that higher sintering temperatures result in increasedshrinkage and lower porosity. Table 1 is a summary of the results:

                  TABLE 1    ______________________________________                   Green Porosity                               Final porosity                                        Change from    #    Temp. (° C.)                   (%)         (%)      Green    ______________________________________    1    675       82          75       -7    2    675       74          63       -11    3    675       75          66       -9    4    675       66          54       -12    5    775       82          68       -14    6    775       74          56       -18    7    775       75          58       -17    8    775       66          42       -24    9    725       82          71       -11    10   725       74          63       -11    11   725       75          63       -12    12   725       66          49       -17    ______________________________________

Example 4

A preferred fabrication process for making metal powder membranes

70 grams of filamentary nickel powder are sifted into a 12.7 cm diametermold from a height of 20 cm to give a 0.7 cm-thick bed. This bed ispressed at approximately 30 kg/cm² (430 psi) to a thickness of 0.4 cmand a density of 1.3 g/cc. The green form has a porosity of 85%. It issintered at 675° C. for 20 minutes in vacuum. After sintering, the greenform (now with a diameter of 11 cm and a thickness of 0.28 cm) ispressed at approx 72 kg/cm² (1032 psi) to a thickness of 0.25 cm and adensity of 2.95 g/cc. The final product has a porosity of 67%.

Although the foregoing invention has been described by way ofillustration and example for purposes of clarity and understanding, itwill be obvious that certain changes and modifications may be practicedwithin the scope of the invention, as limited only by the scope of theappended claims.

I claim:
 1. A high-porosity metal filter comprising:a membrane elementformed by . .sintering.!. .Iadd.increasing the density of .Iaddend.amass of dendritic metal particles .Iadd.and then sintering said mass.Iaddend.in the absence of any extraneous material cohesively binding orsupporting said dendritic particles forming said mass. .; each of saidmetal particles having intertwined appendages and the intertwinedappendages between adjacent sintered dendritic metal particles forming amatrix of substantially interconnected pores;.!. .Iadd.whereby .Iaddend.the porosity of said membrane element . .being defined by saidintertwined appendages and being.!. .Iadd.is .Iaddend.at least 55%. 2.The metal filter of claim 1 wherein said porosity is from about 55% toabout 70%.
 3. The metal filter of claim 1 wherein said metal comprisesnickel.
 4. The metal filter of claim 1 wherein said element is sinteredat a temperature of from about 675° C. to about 725° C.
 5. The metalfilter of claim 1 wherein a substantially uniform low-density bed ofsaid dendtritic metal particles is formed prior to sintering byair-laying said particles into a mold.
 6. A high-porosity membranefilter comprising:a membrane element formed by sintering a mass ofdendritic metal particles in the absence of any extraneous materialcohesively binding or supporting individual dendritic particles formingsaid mass; each of said metal particles having intertwined appendagesand the intertwined appendages between adjacent sintered particlesforming a matrix of substantially interconnected pores within saidmembrane element, the porosity of said element being at least 55%; afilter housing defining a fluid conduit, said housing comprising acasing for retaining said membrane element in said fluid conduit, saidcasing having an anterior and a posterior with said element beinglocated therebetween and being sealably joined to said casing, saidhousing thereby defining a filtered fluid flow path; and means forsealably connecting said casing to a fluid to be filtered.
 7. Thehigh-porosity membrane filter of claim 6 wherein the porosity of saidmembrane element is from about 55% to about 70%.
 8. The high-porositymembrane filter of claim 6 wherein said metal particles are nickelparticles.
 9. The high-porosity membrane filter of claim 8 wherein saidmembrane element is sintered at a temperature of from about 675° C. toabout 725° C.
 10. The high-porosity membrane filter of claim 6 wherein asubstantially uniform low-density bed of said dendtritic metal particlesis formed prior to sintering by air-laying said particles into a mold.11. A method of making a high-porosity metallic membrane elementcomprising the steps of:air-laying a substantially uniform low-densitybed of a sinterable dendritic material into a mold in the absence of anyextraneous material, said mold being suitable for applying compressiveforce thereto, said bed having a density less than the apparent densityof said sinterable dendritic material; compressing said low-density bedto form a green form; and sintering said green form.
 12. The method ofclaim 11 wherein compressing said substantially uniform low-density bedof sinterable dendritic material occurs at a pressure below about 500psi.
 13. The method of claim 11 wherein compressing said substantiallyuniform low-density bed occurs at a pressure below about 1000 psi. 14.The method of claim 11 wherein said sinterable dendritic materialcomprises nickel.
 15. The method of claim 11 wherein said metallicmembrane element has a porosity of from about 55% to about 70%.
 16. Themethod of claim 11 wherein said green form is sintered at a temperatureof from about 675° C. to about 725° C.
 17. The method of claim 11wherein said membrane element is compressed at a second higher pressureafter sintering, thereby imparting additional structural rigidity tosaid element.
 18. The method of claim 11 wherein said membrane elementis cut into individual filter elements of a predetermined size.
 19. Themethod of claim 18 wherein said membrane element is cut by wireelectrical discharge machining. .Iadd.20. The high-pososity metal filterof claim 1, wherein the density of said mass of dendritic particles isincreased by compressing said mass at a pressure below about 500 psi..Iaddend..Iadd.21. The high-porosity metal filter of claim 20, whereinthe density of the mass of dendritic particles of the mass is increasedby applying a pressure to said mass of about 150 psi. .Iaddend..Iadd.22.The high-porosity metal filter of claim 1 wherein the density of themass of dendritic particles is increased by applying compression to saidmass of dendritic particles. .Iaddend..Iadd.23. The high-porosity metalfilter of claim 1, wherein the density of the mass of dendriticparticles is sufficiently increased to form a self-supporting greenform. .Iaddend..Iadd.24. The high-porosity metal filter of claim 1,wherein the porous membrane filter is formed by sintering said dendriticmass of a temperature of up to about 950° C. .Iaddend..Iadd.25. Thehigh-porosity metal filter of claim 1, wherein the density of said massof dendritic particles is increased by compressing said mass at apressure below about 1000 psi. .Iaddend..Iadd.26. The high-porositymetal filter of claim 1, wherein the membrane element is formed byfurther including the step of compressing the mass of dendriticparticles after said mass has been sintered. .Iaddend..Iadd.27. Thehigh-porosity metal filter of claim 26, wherein the membrane element iscompressed at a pressure of about 500 psi. .Iaddend..Iadd.28. Thehigh-porosity metal filter of claim 1, wherein the membrane element iscompressed at a pressure in a range of between about 600 and about 1100psi. .Iaddend..Iadd.29. The high-porosity metal filter of claim 1,wherein the membrane element is compressed at a pressure greater thanabout 1000 psi. .Iaddend..Iadd.30. The high-porosity metal filter ofclaim 1, wherein the pores of said filter are substantiallyinterconnected. .Iaddend..Iadd.31. The high-porosity metal filter ofclaim 1, wherein the average diameter of pores of said filter is in arange of between about 2 and about 10 μm. .Iaddend..Iadd.32. Thehigh-porosity metal filter of claim 5, wherein the dendritic particlesare air-laid through a sieve at least 25 cm above a mold in which saidmass of dendritic particles is formed. .Iaddend..Iadd.33. Thehigh-porosity metal filter of claim 32, wherein the metal comprises ametallic material. .Iaddend..Iadd.34. The high-porosity metal filter ofclaim 33, wherein the metallic material includes nickel..Iaddend..Iadd.35. The high-porosity metal filter of claim 34, whereinthe density of the membrane element is between about 2.75 and 3.0 g/cc..Iaddend..Iadd.36. The high-porosity metal filter of claim 1, whereinthe porosity of said membrane element is greater than about 65%..Iaddend..Iadd.37. The high-porosity metal filter of claim 36, whereinthe porosity of said membrane element is between about 67% and about72%. .Iaddend..Iadd.38. The high-porosity metal filter of claim 36,wherein the porosity of said membrane element is in a range of betweenabout 70 and about 80%. .Iaddend..Iadd.39. The high-porosity metalfilter of claim 6, wherein said mass of dendritic particles is sinteredat a temperature of up to about 950° C. .Iaddend..Iadd.40. Thehigh-porosity metal filter of claim 6, wherein said anterior andposterior define a casing wall, and wherein said membrane element isbonded to said casing wall. .Iaddend..Iadd.41. The high-porositymembrane element of claim 6, wherein the porosity of said membraneelement is between about 67% and about 72%. .Iaddend..Iadd.42. Themethod of claim 11, wherein said green form is sintered at a temperatureof up to about 950° C. .Iaddend..Iadd.43. A method of filteringparticulates from a gas, comprising the step of directing the gasthrough a filter element, said filter element having been formed bysintering, at a temperature of up to about 800° C., a mass of dendriticparticles in the absence of any extraneous material cohesively bindingor supporting said dendritic particles, whereby said filter element hasa porosity of at least about 55%. .Iaddend..Iadd.44. The method of claim43, wherein the filter element through which the gas is directed iscomprised of nickel. .Iaddend..Iadd.45. The method of claim 43, whereinthe filter element through which the gas is directed is formed fromdendritic particles having an average particle size in a range ofbetween about 2 and about 3 μm. .Iaddend..Iadd.46. The method of claim43, wherein the filter element through which the gas is directed definespores having an average diameter in a range of between about 2 and about10 μm. .Iaddend..Iadd.47. The method of claim 43, wherein the filterelement through which the gas is directed has a density in a range ofbetween about 2.75 and about 3.0 g/cc. .Iaddend..Iadd.48. The method ofclaim 43, wherein the filter element through which the gas is directedis formed by sintering said dendritic mass at a temperature in a rangeof between about 675° C. and about 725° C. .Iaddend..Iadd.49. The methodof claim 43, wherein the filter element through which the gas isdirected is formed by air laying said dendritic particles into a mold toform a substantially uniform bed of said dendritic particles prior tosintering. .Iaddend..Iadd.50. The method of claim 43, wherein the filterelement through which the gas is directed is formed by increasing thedensity of said bed of dendritic particles prior to sintering..Iaddend..Iadd.51. The method of claim 43, wherein the filter elementthrough which the gas is directed is formed by compressing said bed ofdendritic particles at a pressure below about 500 psi..Iaddend..Iadd.52. The method of claim 43, wherein the filter elementthrough which the gas is directed is formed by compressing said bed ofdendritic particles at a pressure below about 1000 psi..Iaddend..Iadd.53. The method of claim 43, wherein the gas beingfiltered is a semiconductor process gas. .Iaddend.